U.S. patent number 5,099,762 [Application Number 07/623,286] was granted by the patent office on 1992-03-31 for electrostatic discharge immune electric initiator.
This patent grant is currently assigned to Special Devices, Incorporated. Invention is credited to Thaddeus R. Drapala.
United States Patent |
5,099,762 |
Drapala |
March 31, 1992 |
Electrostatic discharge immune electric initiator
Abstract
An electro-explosive initiator which may be either a coaxial or
floating case type, for example, and which is constructed to be
immune from the effects of electrostatic discharges. The initiator
includes a bridgewire and contact pins for carrying an electric
igniting signal to the bridgewire. The initiator has a discrete
capacitor connected across the contact pins to shunt out the energy
resulting from an electrostatic discharge pulse, and for preventing
such energy from reaching the bridgewire.
Inventors: |
Drapala; Thaddeus R. (Canyon
Country, CA) |
Assignee: |
Special Devices, Incorporated
(Newhall, CA)
|
Family
ID: |
24497495 |
Appl.
No.: |
07/623,286 |
Filed: |
December 5, 1990 |
Current U.S.
Class: |
102/202.1;
102/202.2 |
Current CPC
Class: |
F42B
3/188 (20130101); F42B 3/182 (20130101) |
Current International
Class: |
F42B
3/182 (20060101); F42B 3/188 (20060101); F42B
3/00 (20060101); F42B 003/18 () |
Field of
Search: |
;102/202.1,202.2 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Jordan; Charles T.
Attorney, Agent or Firm: Finkel; Robert Louis
Claims
I claim:
1. An electro-explosive initiator constructed to be immune from
electro-static discharge pulses resulting from ambient
electro-static fields encountered by the initiator, said initiator
comprising:
a casing;
an explosive charge mounted within said casing;
a bridgewire mounted within said casing for igniting said charge in
response to an electric current through said bridgewire;
first and second terminal pins directly connected to said
bridgewire for supplying an electric current to said bridgewire for
igniting said explosive charge;
first and second leads;
first and second splice members respectively connecting said leads
to said first and second pins; and
a discrete capacitor having a capacity substantially in the range
of 0.1 microfarad to 10.0 microfared, said capacitor having first
and second lead wires respectively connected to said leads to said
first and second splice members whereby substantially all of the
energy of an electro-static discharge pulse resulting from an
ambient electro-static field encountered by the initiator is
shunted so as to prevent energy resulting from said discharge pulse
from reaching the bridgewire.
2. The initiator defined in claim 1, in which said lead wires of
said capacitor are resistance welded to said first and second
splice members.
3. The initiator defined in claim 1, in which said capacitor
capacity of the order of 0.33 microfarads.
4. The initiator defined in claim 1, in which said capacitor has a
voltage rating of the order of 50 Volts.
5. The initiator defined in claim 1, and which includes a radio
frequency filter interposed between said first and second terminal
pins to absorb radio frequency signals and prevent spurious
ignition of the initiator by such signals.
Description
BACKGROUND OF THE INVENTION
The invention is concerned with electro-explosive devices or
"initiators", and it has particular application to initiators which
are known to the art as "squibs". The invention is directed to an
improved squib or initiator which is constructed to be immune from
premature firing by static electricity.
As discussed in U.S. Pat. No. 2,802,421, which issued Aug. 13,
1957, the art has long recognized the dangers in the accidental
discharge of electric initiators by static electricity. Accidents,
which at the time of their occurrence seemed without explanation,
have been subsequently traced to the firing of an initiator by a
static electricity discharge. Since conventional ignition
compositions necessarily are highly heat sensitive, a discharge of
relatively high voltage from static electricity is quite capable of
igniting the composition and firing the initiator. The art has
generally considered that accidental firings result from a direct
static discharge from a lead wire to the metallic case of an
initiator in the locus of the ignition composition.
The danger of premature firing due to static discharge is present
in all types of electric initiators. A structure has previously
been proposed in the prior art in which a lead wire is disposed in
contact or almost in contact with the metallic case of the
initiator, at a point removed from the ignition composition, in
order to provide for a discharge of the static electricity from the
lead wire to the case. However, this type of structure offers good
protection from static or firing of the initiator only when the
charge passes down the one lead wire to the case. Little or no
benefit is obtained when the current passes through both lead
wires. Even when both lead wires are so disposed, discharge will
occur from only one wire in many instances, and protection will be
limited as far as heating of the bridge wire is concerned.
A structure has also been proposed in the prior art wherein one or
both of the lead wires are connected to the metallic case of the
initiator by means of semiconductive material outside the locus of
the ignition composition. This construction affords a considerable
improvement in resistance to static discharge. However, it is very
difficult with such a structure to maintain a proper balance of
conductivity that will allow a discharge from both wires to the
case and still have sufficient resistance for protection against
the low voltage currents which attend many commercial operations.
Additionally it has been found that in some instances, the static
resistance of this type of structure diminishes with storage.
Furthermore, such initiators have been known to fire when 10-40
volts from a battery are applied between the lead wires and
shell.
In another proposed structure, conductive material is disposed
about the bared lead wires and extends to the case. This conductive
material acts as a true resistor in that the resistance is low,
normally form 10-100 ohms. The resistance of such a body of
material is similar to that of a regular carbon resistor, being
fairly constant, but subject to variation due to temperature. The
resistance does not change greatly due to passage of current until
the current is sufficient to cause heating. This type of initiator
gives good static protection in most instances. However, it has
been found that in some instances discharge occurs from only one
wire which allows a firing of the initiator by the heating of the
bridge wire. Even more than in the case of semiconductive material,
this structure has a serious deficiency of insulation from case to
lead wires and can be fired in this manner with a very low voltage.
In other words, this structure, while removing a considerable part
of the hazard of static electricity, has introduced an equally
undesirable hazard in the form of undesirably low resistance
between lead wires and case.
In still another structure, it has been proposed to equipt the
bared lead wires with protrusions which extend toward the metallic
case. This structure is usually employed with a matchhead ignition
element which is insulated. However, a discharge usually takes
place from only one wire and a firing of the initiator by the
heating of the bridge wire is thus permitted. In addition, a hotter
spark is obtained when the discharge is directed from localized
points. Even though the protrusions are outside the locus of the
ignition composition, such violent discharges are to be avoided
when the same results can be obtained in a less violent manner.
While all of the initiators described exhibit a measure of
resistance to static electricity discharges, it will be seen that
in each case, the protection from static electricity is not
complete and, in most instances, what protection is obtained is
brought about by a structure which is characterized by low voltage
breakdown.
Ignitors have found widespread use as airbag inflators in motor
vehicles. The initiator commonly used in such applications is a
two-pin squib with a "floating" electrically conductive case. In
such an initiator, the firing signal is applied by way of an
electric circuit located within the case, but insulated therefrom,
the electric circuit being connected to the pins of the squib to
energize a bridgewire within the squib. In the two-pin squib, the
bridgewire is connected between the two-pins rather than between
one pin and the case, as is the case in the coaxial type of
squib.
Specific attempts have been made in the prior art to render the
"floating" case initiator described in the preceding paragraph
immune from electrostatic discharges. These attempts have included
the use of a "leaky" insulator. As described in U.S. Pat. No.
3,783,788, electrostatic charges accumulating between the pins, or
between either pin and the case, are dissipated by a low-level
current flow through the insulator. However, this approach has met
with but limited success. Another attempt in the prior art to
render such initiators immune from electrostatic discharges has
involved the use of Zener diodes connected between the pins and
between each pin and the case. This arrangement also is met with
only limited success, and has proven to be cost prohibitive.
U.S. Pat. No. 4,261,263, assigned to the present assignee,
describes and claims yet another approach to immunity from radio
frequency signals and electrostatic discharges, which has proven to
be highly successful. The initiator described in the discharge of
radio frequency and electrostatic energy, the spark gap being
isolated from the pyrotechnic charge within the initiator.
An objective of the present invention is to provide an improved and
simple initiator which may be of the "floating" case. type or of
the coaxial type, and which is constructed to exhibit high immunity
to electrostatic discharges resulting from ambient electrostatic
fields.
The electrostatic discharge test specifications of m automobile
manufacturing companies requires that a 500 pf capacitor charged to
25 kilvolts be discharged through the initiator by way of a 5
kilo-ohm series resistor. In the case of the floating casing type
of initiator, the test is specified to be conducted in three
formats, namely pin-to-pin, each pin-to-case, and the
short-circuited pins-to-case. The electrostatic discharge test
specification for one major automobile manufacturer requires that a
150 pf capacitor charged to 25 kilvolts be discharged through a 150
ohm series resistor.
The latter test dissipates approximately 10 times the energy in the
initiator as compared with the former test, and this has resulted
in a high number of firings in the lead-to-lead 1 test mode when
the prior art initiators were tested. This is because the energy
dissipated in the bridgewire is comparable with the energy required
to fire the initiator. For that reason, the prior art initiators
for the most part have been found to fail the severe test.
SUMMARY OF THE INVENTION
The present invention provides an initiator which may be the
floating case type or the coaxial type, and which includes a
capacitor connected across its terminal pins. The capacitor serves
to shunt the major part of any electrostatic discharge pulse away
from the bridgewire, thereby substantially reducing the energy
dissipated by the bridgewire in response to such a pulse. The
initiator of the invention has proven to be immune to electrostatic
discharges, even when subjected to the severe test described
above.
It should be pointed out that radio frequency filters have been
used in the prior art to protect initiators from spurious firings
due to exposure to radio-frequency energy an arrangement is
disclosed, for example, in U.S. Pat. 3,343,491. However, the prior
art demonstrates that such filters are normally used to protect the
initiator from the effects of radio frequency signals. It is well
known that the electrostatic discharge is not an alternating
current signal, so that the use of a capacitor to dissipate
electrostatic energy is clearly unobvious. The capacitor has been
found to be operational because the electrostatic discharge is in
the form of a pulse which contains high frequency components due to
its transient nature, and these components travel a low impedance
path through the capacitor. Accordingly, there is no teaching in
the prior art of a firing circuit for an initiator which includes a
capacitor connected across the pins of the initiator for the
purpose of rendering the initiator immune to electrostatic
discharges.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective sectional view of a typical coaxial type
initiator which may be conditioned, in accordance with the
teachings of the present invention to be immune to from
electrostatic discharges;
FIG. 2 is a perspective view of the initiator of FIG. 1 together
with appropriate leads connected to the terminal pins of the
initiator, and including a capacitor connected across the pins, in
accordance with the teachings of the present invention;
FIG. 3 is a schematic representation of the firing circuit of the
initiator of FIG. 2;
FIGS. 4A AND 4B are curves showing the total energy introduced to
the bridgewire of an initiator, such as the initiator of FIG. 1,
and the voltage across the bridgewire and current through the
bridgewire, during a typical test; and
FIGS. 5A and 5B are curves showing the reduction in energy
introduced to the bridgewire, and the reduction in voltage across
and current through the bridgewire during an identical test, but
with the capacitor connected across the pins of the initiator as
shown in FIGS. 2 and 3.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The initiator shown in FIG. 1 includes a body 10 formed of nylon or
other appropriate material. A pair of terminal pins 12 and 14 are
supported within body 10. Terminal pin 12 is connected to one end
of a bridgewire 16, and terminal pin 14 is connected to a header 18
which, in turn, is connected to a grounded case 20. The other end
of bridgewire 16 is connected to the grounded case 20 by way of
header 18. An ignition charge 22 is supported in the casing 20
adjacent to the bridgewire, with the bridgewire being sandwiched
between a ceramic chip 24 and the ignition charge 22. A glass seal
26 is mounted in body 10 adjacent to the ceramic chip 24. The
ignition charge 22 is mounted with a sleeve 28. An output charge 30
is mounted in casing 20 adjacent to the ignition charge 22 but
axially spaced from the ignition charge.
A radio-frequency filter 32 is interposed between the pins 12 and
14 to absorb any radio-frequency signal and prevent spurious
ignition of the initiator by such signals. The pins 12 and 14 are
connected to appropriate lead wires 36 and 38 by conductive splices
40 and 42 formed of brass, or other suitable, material, and which
are clamped to the lead wires and to the respective pins. The
initiator is activated by applying an electric signal to lead wires
36 and 38, the signal being conducted through the pins 12 and 14 to
the bridgewire 16. The bridgewire accordingly heats up, and ignites
the ignition charge 22 which, in turn, ignites the output charge
30.
In accordance with the present invention, and is shown in FIGS. 2
and 3, a capacitor 50 is connected across the pins 12 and 14 of the
initiator, specifically to absorb electrostatic discharges and
thereby protect the bridgewire 16 from spurious ignition due to
such discharges. Specifically, in a constructed embodiment of the
invention, capacitor 60 is a 0.33 microfared ceramic capacitor
whose leads are resistance welded to splices 40 and 42. The voltage
rating of the capacitor is 50 V.
The curves of FIGS. 4A and 4B show the performance characteristics
of the initiator of FIG. 1 without the capacitor 50, and the curves
of FIGS. 5A and 5B show the performance characteristics of the
initiator of FIGS. 2 and 3 with the addition of capacitor 50.
FIGS. 4A and 5A show the total energy supplied to the bridgewire,
and FIGS. and 4B and 5B show the voltage and current across and
through the bridgewire, as a function of time. It will be observed
from the curves that the use of capacitor 50 causes the total
energy supplied to the bridgewire, as well as the voltage and
current across the bridgewire to be substantially reduced.
The curves of FIGS. 4A, 4B and 5A and 5B are the results of tests
conducted on a total of 40 initiators, 10 unprotected and 10 each
with varying values of capacitor 50, namely, 0.1 uf 0.33 uf and 1.0
uf. Each initiator was subjected to 30 consecutive pulses from a
460 uf capacitor charged to 30 kV, and discharged through a 150 ohm
series resistor. Total pulse energy (0.5 KV ) exceeded the severe
test described above by a fact or of 4.4. All ten unprotected
initiators fired, nine on the first pulse and one on the second
pulse. One out of ten units fitted with a 0.15 uf capacitor failed
on the sixth pulse. The remaining nine initiators showed a
significant reduction in bridgewire resistance after testing, an
indication that the bridgewire had seen a substantial electrostatic
discharge pulse. None of the ten units fitted with a 0.33 uf
capacitor fired, and they showed only a very slight reduction in
bridgewire resistance. None of the ten units fitted with a 1.0 uf
capacitor fired, and none showed any change in bridgewire
resistance.
The invention provides, therefore, a simple initiator which
incorporates a capacitor connected across its terminal pin to
render its bridgewire immune from firing in the presence of
electrostatic discharges.
It will be appreciated that while a particular embodiment of the
invention has been shown and describe modifications may be made.
The present invention is intended to cover all such modifications
as set forth in the following claims.
* * * * *